Online grocer Ocado has recorded a slowdown in sales, which the company blamed on the disruption caused by the diamond jubilee celebrations and the London Olympics.

In line with an early warning that it could be hit by these events, Ocado said third-quarter sales rose by 9.9%, to £162.6m, compared with a 12% increase in the first half of the year. The latest figures are to the start of August, so they may not show the full impact from the 2012 Games.

Chief executive Tim Steiner said Ocado's performance had improved since then: "Our customers' ordering patterns have returned to normal after the one-off jubilee and Olympic events, and we are excited by the prospects of the changes we are making to our proposition to shoppers."

These include a "low price promise" to persuade consumers that Ocado – which still relies on upmarket supermarket Waitrose for the bulk of its products – is competitive with rivals. Steiner said the firm was was aiming to boost sales growth for the rest of the year, although he warned that the grocery market and the general economic picture remained challenging.

The company has suffered a series of profit downgrades since its flotation in July 2010. It has been hit by fears of growing competition in its key market within the M25, not least from Waitrose itself, as well as the cost of developing a second distribution centre in Warwickshire, which is scheduled to begin operations in the first quarter of 2013.

Some analysts are concerned that Ocado could breach some of its banking covenants.

Clive Black, an analyst at Shore Capital, repeated his long-standing sell recommendation following the figures: "Ocado is burning through cash, leading to a deceleration in the pace of investment to stay within fulsome banking covenants.

"We remain of the view that there is a reasonable prospect that Ocado shareholders may be asked for further funds, no doubt at a discount to the current price, to fund the development of the business and that bankers, who must be concerned, will place charges and restrictions upon a management where spending capabilities are top notch but returns are distinctly Isthmian League."